Implantable medical prostheses, such as stents, are placed within the body to maintain and/or treat a body lumen that has been impaired or occluded, for example, by a tumor. The stent can be formed of strands of material formed into a tube and are usually delivered into the body lumen using a catheter. The catheter carries the stent to the desired site and the stent is released from the catheter and expands to engage the inner surface of the lumen.
A self-expanding stent can be made of elastic materials. These are held in a compressed condition during catheter delivery by, for example, a sheath that covers the compressed stent. Upon reaching the desired site, the sheath constraining the stent is pulled proximally, while the stent is held in the desired position such that the stent expands.
There are both self-expanding and non-self-expanding stents. The self-expanding type of device is made with a material having an elastic restoring force, whereas a non-self-expanding stent is often made with elastic, plastically deformable material. It is positioned over a mechanical expander, such as a balloon, which can be inflated to force the prosthesis radially outward once the desired site is reached.
In a preferred embodiment, the invention features an implantable medical stent having a low profile during delivery. The stent is a tubular body with a body wall structure having a geometric pattern of cells defined by a series of elongated strands extending to regions of intersection. An example of a stent having a cell shape in accordance with the invention can be found in U.S. Pat. No. 5,800,519, which issued on Sep. 1, 1998, the entire contents of which is incorporated herein by reference. This stent cell structure utilized helically wrapped joints to connect the different strands to form a tubular body.
A limitation on the use of the helically joined stent involved the minimum constrained diameter of the stent during delivery. Because of the helically wrapped joints abutting one another along a given circumference, the minimum constrained diameter of the stent was 9 French (3 mm). For example, the length of the helically wrapped joint for a strand having a diameter of 0.006 inches (0.15 mm) in the constrained position is 0.045 inches (1.1 mm). For a five cell structure having five helically twisted abutting joints, this results in a constrained circumference of 0.228 inches (5.79 mm) with a diameter of 0.072 inches (1.8 mm). However, there are many applications in which it is necessary to achieve a smaller constrained diameter to provide delivery, for example, through smaller lumens within the vascular system, to reduce trauma during percutaneous delivery, or to provide endoscopic delivery through small diameter channels of endoscopes.
To achieve a smaller constrained diameter of 8 French or less, for example, a preferred embodiment of the invention replaces one or more of the helically wrapped joints along any given circumference with a simple crossed joint in which one strand crosses either above or below a second strand. Thus, the strands at a crossed joint can move more freely relative to each other, but this structure reduces the minimum circumference as the length of one or more helically twisted joints has been removed. This can reduce the constrained diameter by 50%.
In another preferred embodiment of the invention, the stent can include a first tubular body made from a first group of strands and a second tubular body surrounding the first tubular body and made from a second group of strands. This type of structure can be used to fabricate a low-profile device having sufficient radial expansion force for a self-expanding stent without a substantial change in foreshortening. This embodiment can include, for example, three or four helically wrapped joints along any circumference of the first and second tubular bodies in which the joints of the two bodies are offset in the constrained state. This embodiment also significantly improves the ratio of the expanded diameter to the constrained diameter.
The strands of the first group can have a different shape, diameter, or material from the strands of the second group such that the inner body has a larger radial restoring force than the outer body and can thereby impart the outward force to the outer body.
In one embodiment, the strands of the inner body can be thicker than the strands of the outer body and can be interleaved with the outer body along the entire length of the stent. In another preferred embodiment, the inner and outer bodies can be interlocked at one or both ends. This can permit the use of a cover between the inner and outer bodies along a certain portion of the stent. The use of the cover can enhance epithialization between the wall of the lumen and the outer body, reduce migration of the stent in certain applications and can prevent tumor in-growth. The cover can also provide a supporting matrix for drug delivery.
In one preferred embodiment, the strands of the stent are woven in a pattern with interlocking joints and skip joints as discussed above. In addition, the adjoining ends of the stent are aligned parallel to each other and laser-welded to secure the adjoining ends of the stent. The welded ends allow the stent to be compressed to a low profile.
In one preferred delivery system, the stent is positioned over an inner shaft and is covered by a composite sheath. The composite sheath can comprise a plurality of materials to provide a variable property such as a graded stiffness along the length of the sheath. In one embodiment the sheath can include a braid or coil between outer and inner sheath layers to provide the longitudinal stiffness and flexibility needed for particular applications. The sheath can have at least a ten percent variation in stiffness along its length and as much as a fifty percent variation with the stiffer section at the proximal end and the least stiff section at the distal end. The sheath can extend coaxially about the inner shaft from the handle connected to the proximal end of the catheter and can be connected to an actuator that is manually operated by the user to slide the sheath relative to the inner shaft.
In one embodiment the inner shaft can include a braided tube, which extends from the proximal handle to a distal position of the delivery system. The inner shaft extends through a lumen of a catheter from the proximal handle to a distance short of the distal end where the catheter ends. The inner shaft can be free-floating within the lumen and receives the stent at the distal end. An outer sheath overlies the stent and the inner shaft and is moved to release the stent using a pull wire which is moved by the proximal handle using a conventional tooth strip attached to a pull wire.
In a preferred embodiment, the inner shaft is formed of steel braided tube encased in a polyimide. For low profile stent delivery systems, where the smaller diameter of the body lumen or the smaller diameter of the endoscope delivery channel necessitate improvements in the push (or pull) strength of the catheter, the use of a braided tube to maintain flexibility and pushability without kinking provides effective delivery of low profile stents.
In the embodiments described above and in other embodiments, a mounting ring can be secured to the inner shaft or braided tube at the stent platform on which the stent is placed. The mounting ring has at least one radial member or ridge which projects towards the outer sheath. The ridge is located preferably at the proximal end of the stent. The ridges extend longitudinally, allowing the stent to be properly positioned while also allowing maximum compression of the stent for minimizing the diameter of the delivery system.